Scrawny individual who has a difficult time putting on mass or gaining weight. Often used as a condescending term among weight-lifters to describe skinny, and thereby weaker, males. In the bodybuilding terminology, the antonym of ectomorph is endomorph - which is a fatter or heavier set individual who conversely has a difficult time losing weight.

Kevin: Look at that ectomorph over there Keith, he's about as skinny as a twig!
Keith: Yeah I would help him out, but I don't care. He can do whatever the F**K he wanna do!

'Ectomorph' traditionally refers to a person who has a fast metabolism and little fat. However, it is more commonly used by those who have little muscle as an excuse. Many skinny people would call themselves 'ectomorphs' or 'hardgainers' but in actual fact they just don't eat enough.

Female: Ewww, why are you so skinny?
Insecure Male: Its because of my genetics. I'm an ectomorph!